Early To Late Miocene History And Evolution Of South Asian Monsoonal Rainfall

During the Middle Miocene,Earth's climate changed from a global warm period(Middle Miocene Climatic Optimum;MMCO)into a colder one with the expansion of the Antarctic ice sheets.This profound climate transition was also a period of drastic changes in global atmospheric circulation.The Miocene history and evolution of the South Asian Monsoon(SAM)however,is not well understood and mainly derived from records of wind strength.For that reason,planktic foraminiferal trace element(Mg/Ca and Ba/Ca)and stable oxygen isotope analyses from National Gas Hydrate Program Expedition Site 01(NGHP-01 Site 01 A;15°18.366N,70°54.192E,water depth 2663 m)were performed in order to reconstruct the regional surface hydrography and thus the hydroclimate in the Eastern Arabian Sea region related to the South Asian monsoon(SAM)during the Early to Late Miocene.Local seawater ?18O estimates(=proxy for sea surface salinity)reveal a notable freshening in the EAS around?13.5-14 Ma.This shift towards lower salinity surface waters is corroborated by an increase in Ba/Ca ratios,indicating enhanced continental runoff.The freshening of the EAS is attributed to an increase in monsoonal rainfall on the coast of West India and the adjacent EAS and an enhanced fluviatile runoff from the Western Ghats.The increase in rainfall in the SAM region coincides with Southern Hemisphere ocean surface water cooling and Antarctic cryosphere expansion,causing an increased cross equatorial pressure gradient and thus strengthening of the monsoonal circulation.